When Should You Switch from a Car Seat to a Booster? A Parent’s Guide

For many parents, one of the most common questions around car seat safety is:

“When should my child move from a harnessed seat to a booster?”

It’s a big milestone — and getting the timing right is essential. Move too early, and your child may not be properly protected. Move too late, and the seat may no longer fit them correctly. This guide will help you understand the key signs that your child is ready to make the transition, and how to choose the safest booster for their next stage.

From Harness to Booster: What’s the Difference?

• Forward-facing harnessed seat: Uses a built-in 5-point harness to restrain the child. The harness spreads crash forces over the strongest parts of the body (shoulders, hips, and chest).

• Booster seat: Raises the child so that the adult seatbelt can be used safely across their body. In a high-back booster, the seat also guides the belt into the correct position and provides side-impact protection.

The harness does the work for younger children, but at a certain point, they’ll be tall and heavy enough to use the adult seatbelt safely — with the help of a booster.

Key Milestones to Watch

1. Outgrowing the Harness

Your child is ready to move on when they reach the maximum weight or height limit for their harnessed seat.

• Under older R44 regulations, forward-facing harnessed seats usually go up to 18 kg (around 4 years old).

• Many newer R129 (i-Size) seats keep children harnessed until 105 cm in height (roughly 4 years old).

💡 Always check your specific seat’s manual — every model has its own limits.

2. Age & Maturity

Even if they meet the size limits, a child also needs the maturity to sit correctly in a booster. They must be able to:

• Sit upright without slumping.

• Leave the seatbelt in place without wriggling out of it.

• Stay in position even if they fall asleep.

For most children, this happens around 4–5 years old — but some may need a little longer.

3. Height & Weight

• Boosters are designed for children from 15 kg (R44) or from 100 cm tall (R129).

• High-back boosters usually cover a wide range, right up to 36 kg or 150 cm tall (around 12 years old).

• This means a single high-back booster can often last a child many years once they’re ready.

Why Not Switch Too Early?

Parents sometimes think moving to a booster is a sign of “growing up” — but switching too soon can be risky.

• A child who slouches or leans out of position won’t be properly restrained by the adult seatbelt.

• The lap belt may ride up into the stomach, or the shoulder belt may cut into the neck.

• A 5-point harness is much better at restraining younger, smaller bodies.

That’s why safety experts recommend keeping children harnessed until they outgrow their seat, and only then moving to a booster.

The Next Step: Choosing the Right Booster

When your child is ready, the safest choice is almost always a high-back booster seat.

• It not only raises the child, but also guides the seatbelt, protects against side impacts, and supports posture on long journeys.

• A cushion might meet the legal minimums for older kids, but it doesn’t offer the same level of protection.

As we explain in our Ultimate Guide to High-Back Booster Seats vs. Cushions, once your child graduates to a booster, choosing the right type makes all the difference in safety.

Final Word: Safety First, Every Time

Switching from a harness to a booster is a big step, but it should be based on size, age, and maturity — not just birthdays. When in doubt, keep your child harnessed until they’ve truly outgrown their seat. And when it’s time, move them into a high-back booster for the safest next stage.

Still unsure which booster is right for your child? Read our complete guide: High-Back Booster Seats vs. Cushions: What Every Parent Needs to Know.